Air species barriers in liquid crystal display devices

ABSTRACT

A device, comprising: a liquid crystal cell comprising liquid crystal material; a stack of layers, including one or more organic polymer semiconductor layers, for electrically controlling one or more optical properties of the liquid crystal material; first and second air species barriers between which both said liquid crystal material and said one or more organic polymer semiconductor layers are located; and a third air species barrier between said liquid crystal material and said one or more organic polymer semiconductor layers; wherein the first and second air species barriers exhibit substantially different transmission rates under the same conditions for at least one air species.

The performance of at least both (a) the liquid crystal material of aliquid crystal display (LCD) device and (b) the organic polymersemiconductor in the stack of layers that defines electrical circuitryfor controlling one or more optical properties of the liquid crystalmaterial, is known to be negatively affected by species present in air;and it is known to encapsulate both these elements together between apair of moisture barriers.

The inventors for the present application have conducted work intofurther improving the performance of OLCD display devices, particularlyin relation to the protection of the LC material and the organic polymersemiconductor against the damaging effects of air species.

There is hereby provided a device, comprising: a liquid crystal cellcomprising liquid crystal material; a stack of layers, including one ormore organic polymer semiconductor layers, for electrically controllingone or more optical properties of the liquid crystal material; first andsecond air species barriers between which both said liquid crystalmaterial and said one or more organic polymer semiconductor layers arelocated; and a third air species barrier between said liquid crystalmaterial and said one or more organic polymer semiconductor layers;wherein the first and second air species barriers exhibit substantiallydifferent transmission rates under the same conditions for at least oneair species.

According to one embodiment, said liquid crystal material is locatedbetween said first and third air species barriers, and wherein saidfirst and third air species barriers exhibit a lower oxygen transmissionrate than said second air species barrier under the same conditions.

According to one embodiment, said device includes a first plasticsupport film supporting said stack of layers, and wherein said secondair species barrier is located between said first plastic support filmand said stack of layers.

According to one embodiment, said second air species barrier comprises ahydrophobic organic polymer.

According to one embodiment, said stack of layers comprises a topconductor pattern, and wherein said third air species barrier layer islocated between said top conductor pattern and said liquid crystalmaterial.

According to one embodiment, said third air species barrier comprisesone or more layers of one or more inorganic insulator compounds.

According to one embodiment, said one or more layers of one or moreinorganic compounds are deposited by sputtering or atomic layerdeposition.

According to one embodiment, said liquid crystal material is locatedbetween said stack of layers and a second plastic support film, and saidfirst air species barrier is located between said second plastic supportfilm and said liquid crystal material.

According to one embodiment, said first air species barrier comprisesone or more layers of one or more inorganic insulator compounds.

According to one embodiment, said one or more layers are deposited bysputtering or atomic layer deposition.

According to one embodiment, said first, second and third air speciesbarriers all exhibit a water vapour transmission rate of less than 0.1 gm⁻² day⁻¹.

According to one embodiment, said first and third air species barriersboth exhibit a lower oxygen transmission rate than said second airspecies barrier.

Using a combination of air species barriers exhibiting differenttransmission rates (as measured under the same conditions) in the LCDdevice facilitates individual tailoring of the protection against airspecies for each of the LC material and the organic polymersemiconductor. For example, if the organic polymer semiconductor is apolymer material whose performance over time is better in the presenceof elemental oxygen, the above-described technique facilitatesprotecting both the LC material and the organic polymer semiconductoragainst moisture, while allowing more elemental oxygen in air to accessthe organic polymer semiconductor than the LC material.

The term “air species barrier” refers to a barrier against thetransmission of one or more air species. In one embodiment, the airspecies barrier is a barrier that has no essential electrical or opticalfunction within the LCD device.

Embodiments of the invention are described in detail hereunder, by wayof example only, with reference to the accompanying drawings, in which:

FIG. 1 shows a first embodiment according to the present invention;

FIG. 2 shows a second embodiment according to the present invention; and

FIG. 3 shows one example of an architecture for the stack of layersdefining electrical control circuitry in FIGS. 1 and 2.

The embodiments described below are for the example of producing anorganic liquid crystal display (OLCD) device, which comprises an organictransistor device (organic thin film transistor (OTFT) device) for thecontrol component. OTFTs comprise an organic semiconductor (such as e.g.an organic polymer or small-molecule semiconductor) for thesemiconductor channels.

The first embodiment described in detail below involves locating outermoisture barriers within the module to which orthogonal polarisers arelater applied. However, the outer moisture barriers are not limited tothese locations. For example, the outer moisture barriers may beprovided outside of the orthogonal polarisers, as in the embodimentillustrated in FIG. 2; and this may be preferable for achieving the bestoptical performance from the liquid crystal display device.

Also, in this first embodiment, the formation of some of the moisturebarriers involves the chemical vapour deposition (e.g. sputtering,atomic layer deposition) of inorganic compounds as part of theproduction process. However, in other embodiments, the formation ofthese moisture barriers may involve the lamination of pre-preparedmoisture barrier films including one or more layers of inorganiccompounds deposited by chemical vapour deposition over a plastic supportfilm.

With reference to FIG. 1, a first embodiment comprises an organicplastic support film 2 (such as e.g. triacetate cellulose (TAC)) towhich is laminated a moisture barrier film 4.

The polymer moisture barrier film 4 exhibits a WVTR of no more thanabout 0.1 g m⁻² day⁻¹, while exhibiting a relatively high oxygentransmission rate higher than the other two moisture barriers mentionedbelow, which as discussed below facilitates exposing the organic polymersemiconductor (discussed below) to elemental oxygen from external air,particularly in the TFT channel regions, discussed below.

Examples of moisture barrier films exhibiting low WVTR while exhibitinga relatively high oxygen transmission rate are hydrophobic organicpolymer films whose chemical composition repels water but whoserelatively low packing density (fraction of unit volume occupied byconstituent particles, i.e. polymer molecules in the case of a polymer))is sufficiently low to offer substantially no resistance to thetransmission of molecular oxygen. One specific example of a hydrophobicpolymer barrier film is a fluoropolymer barrier film such as e.g. aHydroblock® P-series TR film made by Honeywell International Inc. Themoisture barrier film 4 does not include any vapour-deposited inorganiclayer (of the kind used in the other moisture barriers mentioned below)whose relatively high packing density would reduce the oxygentransmission rate.

In this embodiment, fluoropolymer barrier film 4 plays no essentialelectrical or optical function in the LCD device; it is included solelyfor its barrier function. The fluoropolymer barrier film 4 is laminatedcontinuously over at least the whole area of the plastic support filmthat remains in the product device (after any trimming process etc.), asshown in FIG. 4, where 2 a indicates the edges of the plastic supportfilm in the product LCD device. The fluoropolymer barrier film 4 is notsubject to any patterning process within this area, such that thefluoropolymer barrier film 4 does not include any intentional holeswithin this area. For conciseness, FIG. 4 only shows those elementsnecessary to illustrate the different positions of the three barriers 4,8, 16 in relation to the LC material 12 and the stack 6 including one ormore organic polymer semiconductor layers.

The working surface (upper surface in FIG. 1) of the moisture barrierfilm 4 is coated with an organic planarization layer or otherwisetreated to provide a working surface better suited to the formation ofan intricate conductor pattern on the moisture barrier film 4.

Next, a stack of layers 6 defining active matrix circuitry forcontrolling the degree to which the liquid crystal material in eachpixel region rotates the polarisation of light, is formed over themoisture barrier film 4. As discussed below in relation to FIG. 3, theuppermost layer of this stack of layers 6 may be a conductor pattern,such as a common conductor pattern 46 for a fringe-field-switching (FFS)type LCD device.

Over the top conductor pattern 46 is formed a layer 8 of an inorganiccompound such as e.g. aluminium nitride by chemical vapour deposition(e.g. sputtering, ALD). In addition to a low WVTR (no higher than about0.1 g m⁻² day⁻¹, this layer also exhibits a lower OTR than the moisturebarrier film 4 mentioned above, and additionally protects the overlyingliquid crystal material against the ingress of elemental oxygen via thestack of layers 6.

In this embodiment, inorganic barrier layer 8 plays no essentialelectrical or optical function in the LCD device; it is included solelyfor its barrier function. This inorganic layer 8 is also formedcontinuously over at least the whole area of the plastic support film 2that remains in the product device (after any trimming process etc.), asshown in FIG. 4, where 2 a indicates the edges of the plastic supportfilm in the product LCD device. The inorganic layer 8 is also notsubject to any patterning process within this area, such that theinorganic layer 4 does not include any intentional holes within thisarea.

Over this barrier layer 8 is formed an alignment layer 10, which(together with the alignment layer 14 on the opposite side of the LCmaterial) functions to retain the LC material in the desired state (inrelation to how it rotates the polarisation of light) when no additionalelectrical field is electrically generated within the LC material viathe control circuitry defined by the stack of layers 6. A rubbed organicpolymer layer such as a rubbed polyimide layer is one example of an LCalignment layer.

Separately, another layer 16 of an inorganic compound is formed bychemical vapour deposition (e.g. sputtering, ALD) on another organicplastic support film 18 (e.g. TAC), whose working surface (bottomsurface in FIG. 1) may be coated with a planarization layer or otherwisetreated to facilitate the formation thereon of the inorganic barrierlayer 16. This inorganic layer exhibits both a low WVTR (no higher thanabout 0.1 g m⁻² day⁻¹) and a lower OTR than the moisture barrier film 4on which the above-described stack of layers 6 is formed. In theassembly shown in FIG. 1, this inorganic layer 16 protects the liquidcrystal material against the ingress of inorganic barrier layer 16 playsno essential electrical or optical function in the LCD device; it isincluded solely for its barrier function. This inorganic layer 16 isformed continuously over at least the whole area of the plastic supportfilm 18 that remains in the product device (after any trimming processetc.), as shown in FIG. 4, where 2 a indicates the edges of the plasticsupport film in the product LCD device. The inorganic layer 16 is notsubject to any patterning process within this area, such that theinorganic layer 16 does not include any intentional holes within thisarea.

The second alignment layer 14 (e.g. rubbed organic polymer layer such arubbed polyimide layer) is formed over the inorganic barrier layer 16,and then this unit is laminated to the unit comprising the firstalignment layer 10 via a precisely controlled thickness of liquidcrystal material 12, to produce a liquid crystal cell. The thickness ofliquid crystal material is controlled by spacers (not shown). Thespacers may, for example, comprise substantially spherical structuresthat are not an integral part of the either the two units laminatedtogether or may be defined by the topographical surface profile of oneor both of the units laminated together.

The lamination may, for example, comprise a roll-to-roll laminationtechnique in which a carefully controlled volume of liquid crystalmaterial deposited in one or more locations is spread over at least thewhole of the display area by the action of laminating the two unitstogether. Alternatively, the lamination may, for example, comprise firstlaminating the units together and then introducing the liquid crystalmaterial into the space between the two units.

Orthogonal, linear polariser films (not shown) are then applied toopposite sides of the liquid crystal cell. The polariser films may eachcomprise a thin layer of iodine-doped polyvinylalcohol (PVA) supportedon a thicker organic plastic support film. Stretching of the thin layerof iodine-doped PVA causes the PVA chains to align in one direction.

In an alternative embodiment shown in FIG. 2, the outer barriers 4, 16in the embodiment of FIG. 1 are replaced by barriers on the outside ofthe polarisation films 22, 24. A moisture barrier film 20 of the kindlaminated to the plastic support film 2 in the embodiment of FIG. 1, andan encapsulation film 26, comprising one or more inorganic layersdeposited by chemical vapour deposition over a plastic support film, arelaminated to opposite sides of the unit comprising the liquid crystalcell and the orthogonal, linear polarisation sheets 22, 24, such thatthe moisture barrier film (having the higher oxygen transmission rate)is on the opposite side of the stack 6 to the LC material 12. It is alsoa feature of this alternative embodiment that the outer barriers 20, 26extend continuously over the whole area of the plastic support films 2,18 in the product device, as shown in FIG. 5; the outer barriers 20, 26are not subject to any patterning in this area, and do not include anyintentional holes in this area. As shown in FIG. 5 the outer barriers20, 26 extend continuously over the whole area of the plastic supportfilms 2, 8 in the product device, and are joined by sealant (now shown)beyond the edges of the area of the plastic support films 2, 8. Forconciseness, FIG. 5 also only shows those elements necessary toillustrate the different positions of the three moisture barriers 8, 20,26 in relation to the LC material 12 and the stack 6 including one ormore organic polymer semiconductor layers.

One example of an architecture for the stack of layers 6 in FIGS. 1 and2 is shown in FIG. 3, for the example of a FFS-type LCD device. Thestack of layers 6 may have a different TFT architecture, and/or may bedesigned for a different type of LCD device, but the stack of layers 6is characterised in this embodiment by the inclusion of an organicpolymer semiconductor, whose performance over time (as measured by thechange over time in threshold voltage for the TFTs) is improved byexposure to elemental oxygen in the absence of moisture.

Over the fluoropolymer barrier film 4 (or plastic support film 2 in thealternative embodiment of FIG. 2) is formed a first conductor pattern(e.g. a metal pattern), which defines source and drain conductors 30, 32for an array of thin-film transistors (TFTs). Each source conductorprovides the source electrodes for a row of TFTs, and extends to alocation outside the display area for connection to a respectiveterminal of a driver chip. Next, an organic polymer semiconductorpattern 34 is formed to provide semiconductor channels in the regionswhere the source and drain conductors for each TFT are closest. Beforeforming the organic semiconductor pattern, the surface of the firstconductor pattern may be treated to tune the work-function of theconductor pattern to improve the efficiency of charge-injection from theconductor pattern into the organic semiconductor. The surface treatmentmay, for example, comprise forming a self-assembled monolayer (SAM) of asuitable organic material.

An organic gate dielectric 36 (comprising one or more organicinsulating, dielectric layers) is formed over the semiconductor pattern4. A second conductor pattern (e.g. metal pattern) is formed over thegate dielectric 36, and defines an array of gate conductors (e.g. gatelines), each gate conductor providing the gate electrode for arespective column of TFTs, whereby each TFT (pixel) is associated with arespective unique combination of source and gate conductors.

Over the second conductor pattern is formed a further, organic insulator(comprising one or more organic insulating layers) 40; and the resultingstructure is patterned (by e.g. etching through a photoresist mask) toat least define vias extending down to each of the drain conductors. Athird conductor pattern (e.g. metal pattern) is formed over theinsulator 40, and defines an array of pixel conductors 42, each pixelconductor in electrical contact with a respective drain conductor 32.Another organic dielectric (comprising one or more organic dielectriclayers) 44 is formed over the third conductor pattern, and a fourthconductor pattern (e.g. metal pattern) is formed over the dielectric 44.The fourth conductor pattern defines the common conductor pattern foreach pixel in the FFS-type LCD device; the pixel electrode 42 and commonconductor pattern 46 for a pixel region are configured such that anelectric potential difference between the two can induce a change in thestate of the LC material in that pixel region (in relation to how the LCmaterial rotates the polarisation of light).

In the above-described stack of layers from the first conductor patterndefining the source/drain conductors to the conductor pattern definingthe common conductor pattern 46, all of the layers are either patternedand/or comprise organic materials. In contrast, the inorganic, barrierlayers 4, 8, 16 (or encapsulation films 20, 26) are all unpatterned inat least the whole area of the plastic support films 2, 18 that remainin the product device (after e.g. any trimming process etc.), and eachhave lower WVTR than any of the layers of this stack, and moreover havea lower WVTR than any of the plastic support films 2, 18, alignmentlayers 10, 14, and polariser films 22, 24.

In addition to any modifications explicitly mentioned above, it will beevident to a person skilled in the art that various other modificationsof the described embodiment may be made within the scope of theinvention.

The applicant hereby discloses in isolation each individual featuredescribed herein and any combination of two or more such features, tothe extent that such features or combinations are capable of beingcarried out based on the present specification as a whole in the lightof the common general knowledge of a person skilled in the art,irrespective of whether such features or combinations of features solveany problems disclosed herein, and without limitation to the scope ofthe claims. The applicant indicates that aspects of the presentinvention may consist of any such individual feature or combination offeatures.

1. A device, comprising: a liquid crystal cell comprising liquid crystalmaterial; a stack of layers, including one or more organic polymersemiconductor layers, for electrically controlling one or more opticalproperties of the liquid crystal material; first and second air speciesbarriers between which both the liquid crystal material and the one ormore organic polymer semiconductor layers are located; and a third airspecies barrier between the liquid crystal material and the one or moreorganic polymer semiconductor layers; wherein the first and second airspecies barriers exhibit substantially different transmission ratesunder the same conditions for at least one air species.
 2. The deviceaccording to claim 1, wherein the liquid crystal material is locatedbetween the first and third air species barriers, and wherein the firstand third air species barriers exhibit a lower oxygen transmission ratethan the second air species barrier under the same conditions.
 3. Thedevice, according to claim 1, wherein the device includes a firstplastic support film supporting the stack of layers, and wherein thesecond air species barrier is located between the first plastic supportfilm and the stack of layers.
 4. The device according to claim 3,wherein the second air species barrier comprises a hydrophobic organicpolymer.
 5. The device according to claim 1, wherein the stack of layerscomprises a top conductor pattern, and wherein the third air speciesbarrier layer is located between the top conductor pattern and theliquid crystal material.
 6. The device according to claim 5, wherein thethird air species barrier comprises one or more layers of one or moreinorganic insulator compounds.
 7. The device according to claim 6,wherein the one or more layers of one or more inorganic compounds aredeposited by sputtering or atomic layer deposition.
 8. The deviceaccording to claim 1, wherein the liquid crystal material is locatedbetween the stack of layers and a second plastic support film, and thefirst air species barrier is located between the second plastic supportfilm and the liquid crystal material.
 9. The device according to claim8, wherein the first air species barrier comprises one or more layers ofone or more inorganic insulator compounds.
 10. The device according toclaim 9, wherein the one or more layers are deposited by sputtering oratomic layer deposition.
 11. The device according to claim 1, whereinthe first, second and third air species barriers all exhibit a watervapour transmission rate of less than 0.1 g m⁻² day⁻¹.
 12. The deviceaccording to claim 2, wherein the first and third air species barriersboth exhibit a lower oxygen transmission rate than the second airspecies barrier.